Pick what to make under the preview and press Randomize until one looks right. The sliders change that same picture; only Randomize rolls a new one, and Undo brings back one you rolled past.
Everything is drawn as SVG in this browser from the colors on the right, so it stays sharp at any size. Download it as SVG, or as PNG, JPG or WebP at up to 4×.
PNG and WebP keep a transparent background; JPG puts white behind it.